Experimental X-ray and DFT Structural Analyses of M12L8 Poly-[n]-catenanes Using exo-Tridentate Ligands

Despite their potential applications in host–guest chemistry, there are only five reported structures of poly-[n]-catenanes self-assembled by elusive M12L8 icosahedral nanocages. This small number of structures of M12L8 poly-[n]-catenanes is because self-assembly of large metal–organic cages (MOCs) with large windows allowing catenation by means of mechanical bonds is very challenging. Structural reports of M12L8 poly-[n]-catenanes are needed to increase our knowledge about the self-assembly and genesis of such materials. Poly-[n]-catenane (1·p-CT) self-assembly of interlocked M12L8 icosahedral cages (M = Zn(II) and L = 2,4,6-tris-(4-pyridyl)benzene (TPB)) including a new aromatic guest (p-chlorotoluene (p-CT)) is reported by single-crystal XRD. Despite the huge internal M12L8 voids (> 2500 Å3), p-CT is ordered, allowing a clear visualization of the relative host–guest positions. DFT calculations have been used to compute the electrostatic potential of the TPB ligand, and various aromatic guests (i.e., o-dichlorobenzene (o-DCB), p-chloroanisole (p-CA), and nitrobenzene (NBz)) included (ordered) within the M12L8 cages were determined by single-crystal XRD. The computed maps of electrostatic potential (MEPs) allow for the rationalization of the guest’s inclusion seen in the 3D X-ray structures. Although more crystallographic X-ray structures and DFT analysis are needed to gain insights of guest inclusion in the large voids of M12L8 poly-[n]-catenanes, the reported combined experimental/DFT structural analyses approach can be exploited to use isostructural M12L8 poly-[n]-catenanes as hosts for molecular separation and could find applications in the crystalline sponge method developed by Fujita and co-workers. We also demonstrate, exploiting the instant synthesis method, in solution (i.e., o-DCB), and in the solid-state by neat grinding (i.e., without solvent), that the isostructural M12L8 poly-[n]-catenane self-assembled with 2,4,6-tris-(4-pyridyl)pyridine (TPP) ligand and ZnX2 (where X = Cl, Br, and I) can be kinetically synthesized as crystalline (yields ≈ 60%) and amorphous phases (yields ≈ 70%) in short time and large quantities. Despite the change in the aromatic nature at the center of the rigid exo-tridentate pyridine-based ligand (TPP vs TPB), the kinetic control gives the poly-[n]-catenanes selectively. The dynamic behavior of the TPP amorphous phases upon the uptake of aromatic guest molecules can be used in molecular separation applications like benzene derivatives.

. (Top) Experimental powder XRD pattern of ligand 2,4,6-tris-(4pyridyl)pyridine (TPP) measured at room temperature. The sample is the one purchased from iChemicals. The experimental powder XRD pattern fits well with the simulated from the crystal structure of TPP (100 K) reported by Zaworotko and coworkers (CCDC Code: UBEJUK) depicted at the bottom.      Single crystal X-ray structure of 1·Tol recorded at room temperature. a) Asymmetric unit and b) M 12 L 8 nanocage containing six toluene guest molecules. It is important to note that the toluene guest molecule is oriented as expected according the computed MEPs for the p-CT and p-CA guests containing a methyl group (see Figure S7).        In this case the grinding process gives a powder product that can be manipulated well (i.e., transferred from the mortar to the vial). Figure S16. Pictures taken during the grinding of TPP and ZnCl 2 . This can be seen from the final weight. The grinding process gives a powder product that can be manipulated very well (i.e., transferred from the mortar to the vial).   Figure   S2. Importantly, the 100 K structure of 1·p-CT does not have continuous channels.

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One p-chlorotoluene guest molecule in the asymmetric unit is ordered and can be resolved by X-ray crystallography (100 K). The p-chlorotoluene occupancy refine to around 0.69 -0.71 values.

Single crystal preparation of poly-[n]-catenane 1·Tol.
For the 1·Tol single crystal preparation, 15 mg of TPB were dissolved in 5 ml :1 ml of toluene:methanol. The homogenous TPB solution was placed in the bottom of a crystallization tube to which a layer of methanol (3 ml) was stratified. Then a methanolic solution of ZnBr 2 (17 mg dissolved in 2 ml of methanol) was added dropwise. The tube was left for 5 days to stand in the lab. Optical inspection showed large single crystals attached to the walls in the middle area of the solution where the TPB and ZnBr 2 were mixed after diffusing. Figure S7. Single crystal X-ray structure of 1·Tol recorded at room temperature. a) Asymmetric unit and b) M 12 L 8 nanocage containing six toluene guest molecules. It is important to note that the toluene guest molecule is oriented as expected according the computed MEPs for the p-CT and p-CA guests containing a methyl group (see Figure S7).   Neat grinding of TPP and ZnBr 2 : For the neat grinding reaction, 30 mg of TPP were ground with ZnBr 2 (32.767 mg) for 15 minutes using a mortar and pestle. As in the previous case, during the grinding the samples were mixed homogeneously with a spatula to achieve the best reactivity among reactants as after ca. 1 minute of grinding the solid remains attached to the surface of the mortar. The color of the product is white ( Figure S14). Obtained weight after grinding: 58 mg (Yield: 92.25 %). The sample was put in a filter paper in a funnel and further washed with a mixture of methanol (4ml) and chloroform (4ml) and left to equilibrate S17 with atmosphere for 1 day. The weight after washing with methanol and chloroform is 46 mg (Yield 73 %).

Figure S15.
Pictures taken during the grinding of TPP and ZnBr 2 . In this case the grinding process gives a powder product that can be manipulated well (i.e., transferred from the mortar to the vial).
Neat grinding of TPP and ZnCl 2 : For the neat grinding synthesis, 30 mg of TPP were ground with ZnCl 2 (20 mg) for 15 minutes using a mortar and pestle. The grinding procedure was as in the ZnBr 2 and ZnI 2 cases described above. The product is white ( Figure S15).
Obtained weight after grinding: 49 mg (Yield: 98.14 %). The weight after washing with methanol and chloroform is 35 mg (Yield 72.1 %). Figure S16. Pictures taken during the grinding of TPP and ZnCl 2 . This can be seen from the final weight. The grinding process gives a powder product that can be manipulated very well (i.e., transferred from the mortar to the vial).

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Note about the neat grinding reaction: Our estimation of product that cannot be recovered after collecting the mechanochemical product and after washing it is ca. 2-3 mg, which means that the actual yields are slightly higher. S19 Table S1. Crystallographic data of 1·p-CT.